This invention relates to an image formation method of a serial printer comprising a color head and a black head placed in parallel each having a plurality of print elements for completing an image by performing the print operation more than once and an image formation control system for controlling the image formation of the serial printer.
FIG. 2 is an illustration to show a schematic configuration of an example of a general serial printer. In the figure, numeral 1 is a carriage drive motor, numeral 2 is a timing belt, numeral 3 is a recording head, numeral 4 is a carriage, numeral 5 is an ink tank, numeral 6 is a carriage guide, numeral 7 is a cable, numeral 8 is recording paper, numeral 9 is a paper feed motor, and numeral 10 is a feed roll. Here, an ink jet printer is shown as a serial printer example.
The carriage 4, which has the recording heads 3 mounted thereon, slides the carriage guides 6 and reciprocates in the horizontal scanning direction of arrow H in the figure. The carriage 4 is moved by the timing belt 2 driven by the carriage drive motor 1. In the example, two recording heads 3 are arranged in the move direction of the carriage 4.
The cable 7 is introduced into the recording heads 3 for supplying power to a heater in the recording heads and transmitting a signal to a drive circuit. The ink tanks 5 are attached to the recording heads 3 for supplying color inks. A plurality of nozzles are placed in each recording head 3 for jetting supplied ink to the recording paper 8 for recording. The recording paper 8 is driven by the feed roll 10 rotated by the paper feed motor 9 in the arrow P direction in the figure for vertical scanning.
A 2-head printer comprising a color head and a black head placed side by side on a carriage in the horizontal scanning direction each head having a plurality of print elements arranged in the vertical scanning direction as a recording head configuration in a serial printer has been developed as shown in FIG. 3, which is a schematic drawing to show an example of recording heads used with the 2-head printer. In FIG. 3, numeral 11 is a color head and numeral 12 is a black head. For example, in an ink jet system, nozzles are arranged in each recording head longitudinally in the figure. The black head 11 performs black recording with all or some of the arranged nozzles.
The nozzles arranged in the color head 12 are separated into groups in response to the number of used ink colors. Here, three color inks of cyan, magenta, and yellow are used and the nozzles for jetting their respective color inks are arranged in line. In FIG. 3, BK denotes black, C denotes cyan, M denotes magenta, and Y denotes yellow. The nozzle groups for jetting different color inks are spaced from each other for preventing colors from being mixed on the head face. Of course, any other color ink may be used.
In such a recording head, the number of nozzles that can print black differs from that of nozzles that can print a color other than black. To use such a recording head for printing, a black image can be printed with the black head width as a unit, thus can be printed at high speed. However, a color image can be printed only with the 1-color width of the nozzles placed in the color head as a unit at the maximum. Normally, once the color mode is set, even a black area is printed only in color image print units. Thus, if an original with color and black areas mixed contains a black area that can be printed at high speed, high-speed print is not performed and the print speed lowers relatively.
To solve such a problem, whether the print object is color or black is determined and the print method is changed in response to whether the image to be printed is color or black, for example, as disclosed in the Unexamined Japanese Patent Application Publication No. Hei 8-238805. Thus, to print an original with color and black areas mixed, a black area is printed at high speed by making full use of the black head width, so that high-speed print is enabled as a whole.
On the other hand, a method of forming an image by horizontally scanning a single area more than once is used as one of recording methods. The number of horizontal scanning times is called the number of print divisions. If the number of print divisions is two, only 50% print is performed by one horizontal scanning, and 100% print is performed by horizontal scanning twice. Paper is fed a half of the 1-color print width of the color head or the whole print width of the black head between the first horizontal scanning and the second horizontal scanning. FIG. 4 is an illustration of an example of a print image formed by horizontal scanning twice; it shows only one color or black and only several dots. First, in print scanning shown in FIG. 4A, image data is thinned out 50% to prepare print data and the print data is printed. Here, assume that the dots indicated by .largecircle. are printed. After the dots are printed, a relative move in the vertical scanning direction is made a half of the print width of the color. In print scanning shown in FIG. 4B, print data of a pattern complementing the dots printed in FIG. 4A is prepared and printed. Here, the dots indicated by .times. are printed. Then, 100% print is complete as in the area at the middle stage shown in FIG. 4C. Such split print is performed for 100% print by horizontal scanning more than once, whereby misalignment of print scanning joints caused by a paper feed error and dot position misalignment caused by directivity variations in nozzles can be made inconspicuous and adjacent dots printed at the same time can be decreased for lessening bleeding for improving the image quality.
In the Unexamined Japanese Patent Application Publication No. Hei 8-238805, such split print in color mode is described, but such a print method in black mode is not used. To perform such split print in the black mode, the following problem occurs at the mode change time:
FIG. 5 is an illustration of the problem at the mode change time when split print is executed. Here, the number of print divisions is two. In FIG. 5, area A shown on the left is an area that can be printed only in black and area B is an area printed in color. Assume that recording heads exist at the position in the vertical scanning direction shown in (1) in the figure. In the next horizontal scanning, the tips of the heads are moved to line b and horizontal scanning is executed in the state in (2) in the figure. The first horizontal scanning of the area between lines a and b is executed. In this state, horizontal scanning of the area between lines a and b needs to be executed once more. If print is continued intact in the black mode, a relative move of the recording heads is made to the position in the vertical scanning direction shown in (3) in the figure and the second horizontal scanning of the area between lines a and b is executed.
However, at the position shown in (3) in the figure, the top of the area B is exceeded for the color of the color head at the bottom stage in the figure. Thus, if a transition to the color mode is made from the position and an attempt is made to execute color print, the color area between lines b and c (area C) cannot be printed in the color of the color head at the bottom stage in the figure. Thus, when the transition is made from the black mode to the color mode, a blank area as much as the print-impossible area is required.
This also applies when the transition is made from the color mode to the black mode; only one horizontal scanning is executed over the area of a half of the black head width from the end line of the color mode and a desired density cannot be obtained. Thus, a blank area as much as the area is required.